1. Field of the Invention
The present invention relates to a mobile communication system and, more particularly, to an improved mobile communication method and system for supporting a bidirectional real time communication services.
2. Description of the Background Art
A universal mobile telecommunications system (UMTS) is a third generation mobile communication system that has evolved from a standard known as Global System for Mobile communications (GSM). This standard is a European standard which aims to provide an improved mobile communication service based on a GSM core network and wideband code division multiple access (W-CDMA) technology. In December, 1998, the ETSI of Europe, the ARIB/TTC of Japan, the T1 of the United States, and the TTA of Korea formed a Third Generation Partnership Project (3GPP) for the purpose of creating the specification for standardizing the UMTS.
The work towards standardizing the UMTS performed by the 3GPP has resulted in the formation of five technical specification groups (TSG), each of which is directed to forming network elements having independent operations. More specifically, each TSG develops, approves, and manages a standard specification in a related region. Among them, a radio access network (RAN) group (TSG-RAN) develops a specification for the function, items desired, and interface of a UMTS terrestrial radio access network (UTRAN), which is a new RAN for supporting a W-CDMA access technology in the UMTS.
The TSG-RAN group includes a plenary group and four working groups. Working group 1 (WG1) develops a specification for a physical layer (a first layer). Working group 2 (WG2) specifies the functions of a data link layer (a second layer) and a network layer (a third layer). Working group 3 (WG3) defines a specification for an interface among a base station in the UTRAN, a radio network controller (RNC), and a core network. Finally, Working group 4 (WG4) discusses requirements desired for evaluation of radio link performance items desired for radio resource management.
FIG. 1 is a block diagram illustrating a general UMTS architecture. The UMTS is roughly divided into a terminal 10, UTRAN 20 and core network 30.
The UTRAN 20 includes one or more radio network sub-systems (RNS) 25. Each RNS 25 includes an RNC 23 and one or more Node Bs 21 managed by the RNCs.
Node Bs are managed by the RNCs, receive information sent by the physical layer of a terminal 10 (e.g., mobile station, user equipment and/or subscriber unit) through an uplink, and transmit data to a terminal 10 through a downlink. Node Bs, thus, operate as access points of the UTRAN for terminal 10.
The RNCs perform functions which include assigning and managing radio resources, and operate as an access point with respect to the core network 30.
The services provided to the specific terminal 10 is roughly divided into a circuit switched service and a packet switched service. For example, a general voice phone call service belongs to the circuit switched service, while a Web browsing service through an Internet connection is classified as the packet switched service.
In case of supporting the circuit switched service, the RNC 20 is connected to the MSC 31 of the core network 30, and the MSC 31 is connected to a Gateway Mobile Switching Center (GMSC) 33 managing a connection to other networks.
Meanwhile, in case of the packet switched service, services are provided by a Serving GPRS Support Node (SGSN) 35 and a Gateway GPRS Support Node (GGSN) 37 of the core network 30.
The SGSN 35 supports a packet communication going toward the RNC 23, and the GGSN 37 manages connection to other packet switched networks such as the Internet.
An interface exists between various network components to allow the network components to give and take information to and from each other for a mutual communication. A cable interface between the RNC 23 and the core network 30 is defined as an Iu interface.
Connection of the Iu interface to the packet switched area is defined as an Iu-PS, and connection of the Iu interface to the circuit switched area is defined as an Iu-CS.
A radio access interface between the terminal 10 and the UTRAN 20 is defined as a Uu interface.
FIG. 2 is a block diagram illustrating a layered radio interface protocol architecture adopted to the Uu interface in FIG. 1. The radio access interface protocol is vertically formed of a physical layer (PHY), a data link layer, and a network layer and is horizontally divided into a control plane for transmitting control information and a user plane for transmitting data information. The user plane is a region to which traffic information of a user such as voice or an IP packet is transmitted. The control plane is a region to which control information such as an interface of a network or maintenance and management of a call is transmitted.
In FIG. 2, protocol layers can be divided into a first layer (physical layer: PHY: L1), a second layer (data link layer: MAC, RLC, and PDCP: L2), and a third layer (network layer: RRC: L3) based on three lower layers of an open system interconnection (OSI) reference model well known in a communication system.
The first layer provides an information transfer service to MAC and higher layers using various radio transfer techniques.
The first layer is connected to the MAC layer through transport channels (TrCHs), and data are transferred between the MAC layer and the PHY layer through the transport channels.
The MAC layer provides a radio resource and MAC parameter reallocation services.
The MAC layer provides data transfer services to the radio link control (RLC) layer through logical channels, and various logical channels are provided for the kinds of data transfer services as offered by MAC.
Each logical channel type is defined by what type of information is transferred. In general, the control plane information is transferred using control channels and user plane information is transferred using traffic channels.
The RLC layer supports reliable data transmission and performs segmentation and reassembly functions of variable-length upper layer PDUs (RLC SDUs) into/from smaller RLC PDUs.
The RLC SDU delivered from the upper layer is segmented into appropriated size and added by header information so as to be transferred to the MAC layer in the form of RLC PDU. The RLC PDUs are temporally stored in an RLC buffer located in the RLC layer.
The packet data convergence protocol (PDCP) layer is located above the RLC layer. A data stream using a network protocol such as an IPv4 (internet Protocol version 4) or an IPv6 (internet Protocol version 6) can be transmitted effectively through the radio interface of a relatively narrow bandwidth by virtue of the PDCP layer.
For this purpose, the PDCP layer performs a function of header compression and decompression using an RFC2507 protocol or RFC3095 (Robust Header Compression ROHC) protocol defined by the Internet Engineering Task Force (IETF).
With such header compression techniques, only information required for the header part is transmitted so that less control information can be transmitted and thus the amount of data to be transmitted can be reduced.
The RRC layer positioned in the lowest portion of the third layer is defined only in the control plane and controls the transport channels and the physical channels in relation to the setup, the reconfiguration and the release of the radio bearers (RBs).
Here, the RB means a service provided by the second layer for data communication between the terminal 10 and the UTRAN 20, and setting up of the RB means processes of stipulating the characteristics of a protocol layer and a channel, which are required for providing a specific service, and setting the respective detailed parameters and operation methods.
For reference, the RLC layer can be included in the user plane and the control plane according to a layer connected to the upper layer. When the RLC layer belongs to the control plane, the data are received from a radio resource control (RRC) layer. In the other cases, the RLC layer belongs to the user plane.
As shown in FIG. 2, in case of the RLC layer and the PDCP layer, a plurality of entities can exist in one layer. This is because one terminal 10 has a plurality of RBs, and only one RLC entity and only one PDCP entity are generally used for one RB.
The RLC layer will now be described in detail.
The RLC layer can perform functions of segmentation and reassembly for the RLC SDU received from the upper layer. After segmentation and reassembly, the RLC layer can add an RLC header to an RLC payload to construct an RLC PDU.
A header of the RLC PDU may contain a sequence number assigned thereto in the transmitted order of RLC PDUs such that the RLC layer of the receiver checks the sequence number of the received RLC PDU and requests retransmission of the lost RLC PDU from the RLC layer of the transmitter, if any.
There are three operation modes for the RLC layer according to functions required by the upper layer, and the RLC layer processes the RLC SDUs according to the operation mode selected.
The three operation modes are a transparent mode (TM), an unacknowledged mode (UM), and an acknowledge mode (AM).
When an RLC entity operates in TM, the RLC entity does not add any header information to the RLC SDU received from the upper layer.
In general, the RLC entity operating in TM does not use the functions of segmentation and reassembly, and thereby, the RLC SDU received from the upper layer is transmitted as it is received. However, if the segmentation function is configured by upper layers the RLC entity segments the RLC SDU into several RLC PDUs. In the case that the RLC SDU is segmented and transferred, RLC PDUs derived from one RLC SDU are to be simultaneously transferred.
When the RLC entity operates in UM, the RLC entity segments the RLC SDU into UMD PDUs of appropriate size if the RLC SDU is larger than the length of available space in the UMD PDU.
Each RLC PDU includes header information so that the RLC layer of the receiver can restore the RLC SDU from the RLC PDUs, and the header information may indicate a position where the RLC SDU ends or contain a sequence number of the RLC PDU.
However, the RLC entity does not retransmit the lost RLC PDU in the UM, even if the receiver does not receive the RLC PDU. That is, the RLC entity of the receiver does not request retransmission of the RLC PDU when it does not receive the RLC PDU or the received RLC PDU is erroneous, and the RLC entity of the transmitter does not duplicate the RLC PDU for retransmission purpose.
Services that can be supported in UM are a cell broadcast service, a voice over IP (VoIP) using an IP network, etc.
Meanwhile, when the RLC layer operates in AM, the RLC entity supports retransmission of RLC PDU when transmission failure occurs.
Whether or not the RLC PDU has been successfully transmitted can be determined by checking the sequence number in the header information of the RLC PDU. If RLC PDU has been lost or erroneous, the RLC entity of the receiver transmits status information (status PDU) indicating the sequences numbers of the lost or erroneous RLC PDU to the transmitter.
When the RLC layer is operated in AM, various timers and counters are defined for an effective retransmission of packets. The timers can be driven after a specific RLC PDU is transmitted, and if no acknowledgement is received in a predetermined time, the RLC entity discards duplicate of RLC PDU and performs a procedure scheduled for this case.
The counter increases by 1 whenever the RLC PDU is transmitted. If no acknowledgement is received in response to RLC PDU even after the counter exceeds a predetermined value, the RLC layer discard the duplicate of the RLC PDU and performs a procedure scheduled for this case.
The RLC entities of the transmitter and receiver set a range of sequence numbers of the RLC PDUs to be transmitted and received, and defines a transmission and reception windows on the basis of the range.
The RLC entity of the transmitter can transmit only the RLC PDUs as much as a window size of the transmission window and the RLC entity of the receiver can adjust or update the window size of a transmission window according to status information to be sent to the transmitter.
The RLC entity of the receiver receives the RLC PDUs as much as the window size of the reception window and discard the RLC PDUs beyond the window size of the reception window.
FIG. 3 is a block diagram illustrating RLC layer of the layered radio interface protocol architecture of FIG. 2.
As described above, a plurality of RLC entities can be activated in the RLC layer, and each RLC entity operates in one of TM, UM, and AM.
When the RLC entity operates in TM or UM, the data transfer is unidirectional as shown in FIG. 3. That is, one RLC entity can only transmit or receive the data in TM or UM, because the retransmission function is not supported in TM or UM.
On the other hand, when the RLC entity operates in AM, the data transfer is bidirectional. This means that the peer AM RLC entities utilizes status information which reports sequence numbers indicating the lost PDUs or erroneous PDUs. That is, the AM RLC entity can simultaneously transmit and receive the data, which means that the AM RLC entity can receive the status information from the receiver while it transmits packets to the receiver.
In detail, since the AM RLC entity includes both a transmission (Tx) module and a reception (Rx) module, it is not defined as the term of a transmission RLC entity or a reception RLC entity like in TM or UM.
In addition, generally, one RB is mapped to one RLC entity, an RB service can be bidirectional or unidirectional according to an operation mode of the RLC entity of the lower layer.
How the packets (RLC PDU) are transferred in respective modes will be described hereinafter in more detail.
In case of TM or UM, the RLC entity of the transmitter does not support retransmission function such that the peer RLC entity of the receiver transfers the packets to an upper layer upon receiving it. However, in case of AM, the AM RLC entity supports an in-sequence delivery function in that packets are sequentially delivered to the upper layer, so that processing delay occurs for reordering the received packets in the order of the transmitted sequence.
The in-sequence delivery function refers to a function of delivering RLC PDUs containing the RLC SDU data to the upper layer in the order that the RLC entity of the transmitter has transmitted them. The RLC entity of the receiver acknowledges successful reception or requests retransmission of the missing PDUs by sending one or more status PDUs to the AM RLC peer entity through its transmitting side. Once a complete RLC SDU has been received, the associated PDUs are reassembled and then delivered to the upper layers through an AM service access point (AM SAP).
Meanwhile, in order to support an effective real time packet transmission, the PDCP layer is defined for the packet switching (PS) domain. Every PS domain radio access bearer (RAB) is associated with one radio bearer (RB), which is in turn is associated with one PDCP entity. Each PDCP entity is associated with one RLC entity.
Every PDCP entity uses zero, one, or several different header compression protocols. Here, the Robust Header Compression (ROHC) protocol is exemplary adopted as a header compressor.
The ROHC is generally used to compress or decompress header information of the Real-time transport protocol/User Datagram Protocol/Internet Protocol (RTP/UDP/IP) packet at the transmitting and receiving entity, respectively.
The RTP/UDP/IP packet refers to a packet containing header information added thereto while the user data passes the RTP, UDP, and IP. The packet header includes various information required for routing to the destination and recovering the transmitted data at the receiver.
The RTP protocol is used to supplement a problem when real time traffic such as a Voice over IP (VoIP) and streaming service is transmitted using the UDP/IP protocol layers. The UDP is one of transport layer protocols over IP and supports connectionless data transfer service unlike the Transmission Control Protocol (TCP) which supports connection-oriented service with the retransmission or flow control functions.
IP is a network layer protocol in terms of OSI reference model and is responsible for moving data packet from node to node based on a destination IP address contained in the packet header. The IP supports best effort delivery service so as to try to forward the packets to the destination but not guaranteed successful delivery.
ROHC operates based on the fact that there is significant redundancy between header fields, both within the same packet header but in particular between consecutive packets belonging to the dame packet stream. By sending static field information only initially and utilizing dependencies and predictability for other field, the header size can be significantly reduced for most packets.
For reference a RTP/UDP/IP packet has an IP (IPv4) header of 20 octets, a UDP header of 8 octets, and an RTP header of 12 octets for a total of 40 octets. With IPv6, the IP header is 40 octets for a total of 60 octets. The size of the payload depends on the coding and frame sizes being used and is as low as 15 to 20 octets.
From these numbers, the need for reducing header sizes for efficiency is obvious. Using the ROHC, the header size can be significantly reduced as much as 1 to 3 octets.
The ROHC has three modes of operation, called Unidirectional mode (U mode), Bidirectional Optimistic mode (O mode), and Bidirectional Reliable mode (R mode).
When the ROHC operates in U mode, packets are sent in one direction only, i.e., from compressor to decompressor. On the other hand, when the ROHC operates in O or R modes, packets are sent in two directions, i.e., a feedback channel is used to send error recovery requests and acknowledgement of significant context updates from decompressor to compressor.
O mode aims to maximize compression efficiency and sparse usage of the feedback channel so as to reduce the number of damaged headers delivered to the upper layers due to residual errors or context invalidation.
R mode aims to maximize robustness against loss propagation and damage propagation, i.e., minimizes the probability of context invalidation, even under header loss/error burst conditions.
FIG. 4 is a block diagram for illustrating peer-to-peer communication between RLC entities operating in UM.
Since the ROHC compressor and decompressor of PDCP peer entities communicates through a unidirectional link in U mode, each PDCP entity at the transmitter and receiver is mapped to one TM or UM LRC entity.
In FIG. 4, a receiver (UTRAN or UE) and a transmitter (UTRAN or UE) communicate through a Uu interface. A PDCP entity at the transmitter is mapped to a transmit UM RLC (Tx UM RLC) entity through a UM SAP and operates a transmit ROHC (Tx ROHC) in U mode. Also, a peer PDCP entity at the receiver is mapped to one receive UM RLC (Rx UM RLC) entity through a UM SAP.
When a PDCP SDU is received from upper layers, the PDCP entity at the transmitter performs header compression using the Tx ROHC upon reception of the PDCP SDU and submit the PDCP PDU to the Tx UM RLC entity through the UM SAP in the sequence received from the upper layer. On the other hand, when the PDCP entity at the receiver receives the PDCP PDU from the Rx UM RLC entity through the UM SAP, it performs header decompression of the PDCP PDU using the Rx ROHC to obtain the PDCP SDU and delivers the recovered PDCP SDU to the upper layer in the order received from the UM RLC entity.
When the PDCP entities at the transmitter and receiver are mapped to respective Tx and Rx TM RLC entities, the transmitter and receiver operate in the similar manner as in UM.
FIG. 5 is a block diagram illustrating a PDCP entity-RLC entities-mapping structure in which the RLC entity operates in AM.
Unlike UM and TM RLC entities, the AM RLC entity can be configured to utilize one or two logical channels so as to transmit and receive at the same time. Accordingly, the AM RLC entities at the transmitter and receiver have the same structure and the AM RLC entity at the transmitter will be exemplary described hereinbelow.
In FIG. 5, a PDCP entity is mapped to an AM RLC entity through an AM SAP. The PDCP entity operates in O mode or R mode (O/R mode) and also the AM RLC entity operates a Tx RLC module and a Rx RLC module, which means that the PDCP entity activates a Tx ROHC module and an Rx ROHC module.
When a PDCP SDU is received from upper layers, the PDCP entity performs header compression using the Tx ROHC module upon reception of the PDCP SDU and submit the PDCP PDU to the Tx RLC module of the AM RLC entity so as to transfer to a transmit side logical channel. On the other hand, when an RLC PDU is received through a receive side logical channel, the Rx RLC module of the RLC entity processes the RLC PDU and then delivers the RLC SDU (PDCP PDU) to the Rx ROHC module of the PDCP entity through the AM SAP. The Rx ROHC module performs header decompression of the PDCP PDU and delivers the PDCP SDU to the upper layer in the order received from the AM RLC entity.
In order for the ROHC to efficiently operate, PDCP PDUs need to be quickly transferred from the RLC entity to the PDCP entity. In this respect, the PDCP entity efficiently operates when the PDCP entity is mapped to the TM/UM RLC entity since the RLC entity delivers the RLC SDUs to the PDCP entity upon receiving the RLC SDUs (PDCP PDUs).
However, when the PDCP entity is mapped to one AM RLC entity, the PDCP entity can not operate well, (i.e., in real time) since the AM entity always operates the retransmission function, in which the RLC PDUs can not be delivered to the PDCP entity until a complete RLC SDU has been received.
Actually, the length of the radio frame specified in UMTS is 10 ms, the time taken by the radio frame to reach the receiver is over 50 ms in consideration of the propagation delay and processing delay at the transmitter and receiver.
Typically, maximum tolerable delay time for supporting the voice telephony or streaming services is 80 ms. Accordingly, if a packet belonged to the radio frame is required to be retransmitted only one time, the total delay for delivering the packet to the upper layer exceed the maximum tolerable delay time. Thus, mapping the PDCP entity operating the ROHC in O/R mode to the AM RLC entity results in degradation of the real time service quality.
Furthermore, the data communication method has a drawback in that since the one PDCP entity can be mapped to only one TM/UM RLC entity, which operates in only one direction, for supporting real time services, it is impossible to support real time bidirectional services.